1. Field of Invention
This invention relates to an apparatus for mounting and positioning laser mirrors. More particularly, this invention relates to an apparatus for wide bandwidth positioning of laser mirrors in tip and tilt about two orthogonal axes and subwavelength planar translations normal to the mirror surface.
2. Description of the Prior Art
As laser technology has become more sophisticated, the need for more compact and accurate laser positioning devices has greatly increased. Conventional mirror positioning devices utilize various combinations of gears, bearings and gimbals. The accuracy of these conventional devices is limited by mechanical problems such as backlash, which causes unacceptable errors in response and predictability. Stiction, i.e., the difference between the dynamic and static friction with regard to sliding surfaces, also has a substantial effect on the accuracy of conventional mirror positioning devices. Once the combined static force of the precision gimbals and geared actuators is overcome, these devices tend to overreact and move beyond the desired setting. These errors have been compounded by the requirements, in modern laser systems, of not only tip and tilt adjustments of the mirror, but also adjustments of the mirror in the direction normal to the surface of the mirror (translation). Adjustments in this direction are needed if it is desired to change the optical path length of the reflected laser beam. If precise enough, the positioning apparatus can provide subwavelength adjustments of the reflected beam thereby allowing, for example, phasing of an array of originally separate laser beams to form a single coherent beam. However, for a compact mirror positioning device, placing the center of rotation for tip/tilt motions about the center of the mirror's surface is mechanically difficult to achieve. When the center of rotation for tip/tilt is not on the mirror's surface, a translation is induced when the mirror is rotated thereby affecting the position of the mirror in the direction normal to the surface of the mirror.
Perturbations arising from acoustical, thermal, vibrational and maneuvering loads on the adjusting apparatus require a tip/tilt range on the order of 1 milliradian, a translational range on the order of 10 micrometers, and an operating bandwidth in both tip/tilt and translation from direct current to several kilohertz (Khz). To date, a mirror positioning apparatus capable of satisfactorily performing these functions within these ranges has not been demonstrated.
The utilization of actuators formed from stacks of piezoelectric wafers is desirable for laser mirror adjustment devices because of the absence of gears and bearings and consequently the elimination of backlash and stiction. However, translation of the mirror is very difficult to achieve without creating a slight tip or tilt of the mirror face. The reason for this difficulty is that in order to achieve tip/tilt free translation designers have been required to rely upon the concurrent actuation of at least three spaced apart piezoelectric actuators which define the translating plane for the mirror. A lead or lag in any of these actuators results in dynamic (real time) tip or tilt of the mirror, and actuation non-linearity in any of these actuators results in static tip or tilt of the mirror. A conventional piezoelectric translator is manufactured by Burleigh Instruments, Inc. and patented under U.S. Pat. No. 3,902,084.